1. Field of the Invention
The invention related generally to the field of electronic displays and more particularly to Color Liquid Crystal Displays (LCDs).
2. Background of the Related Art
LCDs operate on the principle that an electric field, when applied to a LCD pixel, will cause the liquid crystals in the LCD pixel to move or rotate. A LCD has an array of LCD pixels (i.e., pixel array), the amount of light which is passed through each LCD pixel is a function of the amount of rotation of the liquid crystals in the LCD pixel. For example, each LCD pixel is constructed such that minimum amount of light passes through the liquid crystals when the voltage (therefore the electric field) applied to the LCD pixel is zero. Typically, in active matrix LCD panels, the rotation of liquid crystal in each LCD pixel is controlled by applying a row voltage and a column voltage during the scanning of the LCD in a scan mode well known in the art. As the LCD is driven in the scanned mode, a LCD pixel at the intersection of the currently selected row and column is rotated based on a video signal to produce the image displayed on the LCD. In a typical example, while the LCD pixel is selected, the row voltage is constant and the column voltage is determined by the pixel based digital data (i.e., the digital form of the video signal) controlling the LCD pixel. The transfer function relationship between the digital data to the analog voltage (i.e., the analog form of the video signal, also called the column voltage) applied to the column of the LCD pixel in the pixel array is called the gamma transfer function by those skilled in the art. The gamma transfer function is column based and is controlled by the circuitry in the column drivers based on externally supplied gamma reference voltages.
The array of LCD pixels in the LCD are constantly lit by a backlight. The constancy of the backlight removes the type of flicker commonly found in CRT (cathode ray tube) screens due to phosphors pulsing with each refresh cycle. Instead, as illustrated by a LCD pixel (100) shown in FIG. 1, the liquid crystals (101) is sandwiched between an upper plate electrode (102) and a lower plate electrode (103) with grooves (104) cut in orthogonal directions. These grooves (104) influence the electric field (not shown) between the upper plate electrode (102) and the lower plate electrode (103) to align the LCD crystals (101) to form channels (105) for the backlight (not shown) to pass through the liquid crystals (101) to the front of the LCD.
As described above, the amount of light emitted through the LCD pixel (100) depends upon the orientation of the liquid crystals (101) in the LCD pixel (100) and is proportional to the voltage (106) applied to the LCD pixel (100). FIG. 2 shows a pixel element (200) including the LCD pixel (100) and the driving circuit in a pixel array of an exemplary LCD (not shown). The LCD pixel (100) in FIG. 2 is shown in a schematic form representing the structure of the LCD pixel (100) shown in FIG. 1. The driving circuit includes a switch (205) (e.g., a transistor switch) and conductors carrying the column voltage (204) and row voltage (207). The lower plate electrode (103) is typically connected to a common node across the pixel array. The voltage at this common node is commonly called Vcom (202). The upper plate electrode (102) is connected to the switch (205). The LCD pixel (201) is generally associated with a capacitance (203). The row voltage (206) is applied to the gate (206) of the switch (205) and controls the conductivity of the switch (205). The switch (205) in turn applies the column voltage (204) to the LCD pixel (100) as controlled by the row voltage (207) through the gate (206). The row voltage (207) and the column voltage (204) are typically applied across a grid of conductors overlaying the pixel array of the LCD.
As the pixel element (200) is selected during the scanning of the LCD, the gate (206) is driven by the row voltage (207) with a voltage swing, for example, from −5V to 20V. The video source driving the LCD supplies a stream of pixel based digital data (i.e., the digital form of the video signal) as the pixel array is scanned. The pixel based digital data is translated into analog voltage (i.e., the analog form of the video signal) carrying the video signal, for example, with an analog video voltage swing ranging from 0V to 10V. The analog video signal is applied as the column voltage (204) during the scanning of the LCD. The intensity information represented by the digital data is realized as the video signal is applied across the LCD pixel. In some examples, the common node Vcom (202) is connected to the backplane of the LCD panel, which is held at ground voltage (i.e., 0V). While this configuration is functional, the LCD panel lifetime may be reduced. One such mechanism that reduces LCD panel lifetime is explained here. As shown in FIG. 1, with Vcom (202) being held at ground and the voltage (106) across the LCD pixel (101) varies from 0V to 10V, there is a substantial average DC voltage level of 5V being applied across each LCD pixel (101). This average DC voltage level causes charge storage, or memory effect of the LCD pixel (101) known to those skilled in the art. In the long term, this memory effect degrades the LCD pixel (101) by electroplating ion impurities onto an electrode of the LCD pixel (101). This contributes to image retention problem, commonly known as a sticking image.
As shown in FIG. 1, the structure of the LCD pixel (100) is symmetrical. The amount of liquid crystal rotation is determined by the magnitude of the voltage (106). For example, in a normally black LCD panel, the pixel element (200) is constructed such that the LCD pixel (100) has minimum brightness when the voltage (106) is zero. Other common configurations include a normally white LCD panel, in which case maximum brightness is achieved when the voltage (106) is minimum (e.g., zero). In the normally black configuration, the voltage (106) applied to the liquid crystals (101) can have either a positive or a negative polarity with same magnitude to align the liquid crystals (101) to produce nominally the same brightness for the LCD pixel (100). It is known in the art to capitalize on this aspect by connecting the lower plate electrode (103) through the common node Vcom (202) to a voltage generator circuitry to set the Vcom (202) at the midpoint of the video signal voltage swing (e.g., 5V in the middle of 0V to 10V). Accordingly, the LCD pixel (101) in the pixel element (200) will have a nominal minimum brightness when the column voltage (204) carrying the video signal is driven to the Vcom (202) voltage level (e.g., 5V). In this configuration, the video signal carried by the column voltage (204) is converted to drive the voltage (106) in a bipolar format such that the voltage of the upper plate electrode (103) swings 5V above and 5V below the common voltage Vcom (202) (e.g., 5V) of the lower plate electrode (103). The converted video signal produces full brightness for the LCD pixel (100) by driving the voltage (106) to opposite polarities in alternating positive and negative fields of the scan mode. This configuration creates a net zero average DC voltage level on the LCD pixel (100) and eliminates the aging and image retention issues.
However, it is known in the art that a LCD panel in this configuration will flicker (i.e., producing alternating light intensities) due to manufacturing variations. For example, the column voltage (204) to produce minimum brightness for the LCD pixel (100) may be 5.5V instead of 5V due to manufacturing variations in the construction of the LCD panel, such as variations in the geometries of the pixel array (not shown), the conductor grid (e.g., carrying the column voltage (204) and/or the row voltage (207)), the pixel element (200), the LCD pixel (100), the driving circuitries, etc. If the column voltage (204) swings between 0V and 10V, the effective full-scale voltage for the video signal in the bipolar format will be different between the positive and negative fields. In one field, the effective full-scale voltage will be 4.5V and in the other field, the effective full-scale voltage will be 5.5V. This difference in effective full-scale voltages translates to a difference in brightness between the positive and negative fields, which is typically experienced as flicker (i.e., light pattern of alternating intensities).
Due to the variations in the construction of each LCD panel through the manufacturing process, while the Vcom (202) is held at the midpoint of the analog video voltage swing, the column voltage (204) to produce minimum brightness for the LCD pixel (100) can differ from panel to panel or across a single panel. Original Equipment Manufacturers using the LCD panel as their system component must therefore adjust each of the panels to eliminate flicker. For LCD with a small screen size where the backplane can be considered a low-impedance ground, a single potentiometer can be added for common voltage adjustment, such as the adjustment of Vcom (202) to compensate for the variation. Traditionally, this is achieved by using mechanical potentiometers and the adjustment is labor intensive. Furthermore, this adjustment can only be made at one gray scale level of the video signal. For example, a flicker video pattern corresponding to a specific gray scale level is displayed on the LCD, and the potentiometer is adjusted until the flicker is minimized. It is known in the art that the required adjustment in Vcom (202) will be different at each gray scale level, therefore adjusting the Vcom (202) at only one gray scale level is a compromise that still results in flicker at other gray scale levels. Since the Vcom (202) is a common voltage for the video signal at all gray scale levels, using Vcom trimming cannot eliminate flicker throughout the entire gray scale range of the video signal.